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What Is a Contact Angle?

The contact angle is a fundamental concept that describes the angle formed at the interface between a liquid and a solid surface. It provides valuable insights into how liquids interact with different materials, influencing phenomena such as wetting and adhesion. By understanding the contact angle, scientists and engineers can manipulate these interactions to design advanced coatings, adhesives, and surface-related applications.

Moreover, the contact angle plays a crucial role in processes like vacuum plasma etching, where it helps ensure the proper adhesion of photoresists during the fabrication of electronic devices.

This article explores the significance of the contact angle and its application in etching processes, particularly in relation to Hexamethyldisilazane (HMDS) priming and optimizing the treatment protocol.

What Does the Term "Contact Angle" Mean?

A diagram showcasing different contact angle measurements

The contact angle helps us understand how the liquid interacts with the solid surface. It tells us how well the liquid spreads out or "wets" the surface. If the contact angle is small, it means the liquid spreads out easily and wets the surface well. On the other hand, if the contact angle is large, it means the liquid tends to form droplets and doesn't spread out as much.

Contact angles are influenced by various factor, such as the type of liquid and solid involved, the temperature, and the cleanliness of the surface. It's important to note that the contact angle can vary depending on whether the liquid is advancing or receding on the surface. This is known as contact angle hysteresis.

The contact angle depends on the type of liquid and solid involved, as well as the shape and roughness of the surface. There are different types of contact angles depending on whether the measurement is taken when the droplet is standing still (static), moving (dynamic), or when the surface is not perfectly smooth (roughness corrected).

By studying the contact angle, scientists and engineers can understand and manipulate how liquids interact with different surfaces. This knowledge has applications in various fields, such as materials science, chemistry, and engineering, where controlling wetting properties is important for designing coatings, adhesives, and other surface-related applications.

What Is Contact Angle Used For?

Let’s start with two terms we’ll need to know before moving forward: Photoresist and Hexamethyldisilazane (HMDS).


Photoresist is a light-sensitive material used in the process of lithography, which is a technique for creating patterns on surfaces. It is commonly used in the fabrication of microchips, printed circuit boards, and other electronic devices.

Think of photoresist as a special type of coating or film that can undergo changes when exposed to light. It is typically applied to a surface, such as a silicon wafer, and then exposed to a patterned light source, often through a mask or a photomask. The light causes a chemical reaction in the photoresist, altering its properties in the exposed areas.

In simpler terms, photoresist is like a photosensitive coating that can be "printed" with light. It allows precise patterns to be created on a surface, enabling the production of intricate structures and circuits in the manufacturing of electronic devices.

Hexamethyldisilazane (HMDS)

Hexamethyldisilazane (HMDS) priming is a pre-treatment process used in the manufacturing of electronic devices, particularly during the lithography process. It involves applying a thin coating of a chemical called hexamethyldisilazane to the surface of a substrate, such as a silicon wafer, before further processing.

The purpose of HMDS priming is to modify the surface properties of the substrate to enhance the adhesion of subsequent layers or materials. HMDS is a hydrophobic (water-repellent) compound, so when applied to the surface, it forms a thin, uniform coating that makes the surface more resistant to the absorption of moisture or water.

HMDS priming is a step where a special chemical coating is applied to the surface of a substrate to improve the adhesion of other materials that will be added later. It makes the surface more resistant to water absorption, which helps the subsequent layers stick better and prevents any issues that could arise due to moisture or water presence.

How Is the Contact Angle Used in Etching?

In the process of etching, the contact angle is used to ensure proper adhesion of the photoresist, which is crucial for the successful patterning and manufacturing of devices such as circuit boards.

Before applying the photoresist, a step called HMDS priming is commonly performed. This priming helps improve the adhesion between the photoresist and the silicon wafer. One of the challenges is the presence of a thin layer of water on the wafer's surface due to ambient humidity. This layer of water can negatively affect the adhesion of the photoresist and lead to delamination during subsequent development or etching processes.

To address this issue, a dehydration bake is performed to remove the water layer. After the dehydration bake, a hydrophobic HMDS coating is applied to the surface. This coating makes the surface more water-repellent (hydrophobic) and reduces the risk of water absorption.

Here's where the contact angle comes into play. Water contact angle measurements are used to determine the effectiveness of the HMDS treatment and to optimize the treatment protocol. By measuring the contact angle, researchers and engineers can assess how well the water droplet spreads or beads up on the treated surface. A higher contact angle indicates better hydrophobicity, meaning the surface is less prone to water absorption and better prepared for the subsequent photoresist application.


In conclusion, the contact angle is a measurement that helps us understand how a liquid interacts with a solid surface. It provides insights into the wetting behavior of the liquid, whether it spreads out easily or forms droplets. By studying the contact angle, scientists and engineers can manipulate the interaction between liquids and surfaces, which has practical applications in fields such as materials science, chemistry, and engineering.

Feel free to contact SCI Plasma for assistance and advice regarding contact angle, etching, and other related topics. Their team of experts has extensive knowledge and experience in these areas, and they can provide valuable guidance to enhance your project and maximize its capabilities. Don't miss out on the opportunity to tap into their expertise.


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